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The Cadmiologia of Johann Gottlob Lehmann: A Sourcebook for the History of Preindustrial Glass Furnaces in Central Europe

All About Glass

Glass furnaces are built to last. Self-destruction, however, seems to be one of their regrettable characteristics. The structure is consumed by high temperatures that no material can withstand indefinitely. Although the life spans of furnaces may have varied considerably, a report of 1649 suggests that it was not unusual to replace them every four to five months.1 Therefore, the chances of excavating substantial remains of a glass furnace are less than slim, and archeological evidence is usually limited to the basic foundations with the ashpit and, less often, to some traces of the sieges on which the crucibles were positioned. In order to interpret these finds, and to come to a better understanding of the history of glass furnaces, further sources of information are required.

Contemporary images and descriptions offer desired evidence, and authors such as Heinrich Maurach and Robert J. Charleston succeeded in compiling an impressive amount of such data.2 Indeed, the quantity of information seemed promising enough for Heinz Horat to envision a detailed typology of glass furnaces, with the ultimate goal of reconstructing historical furnaces to their original scale and full functionality, and thereby achieving a better understanding of the corresponding glass technology.3 However, Horat himself is the first to concede that the existing material provides only incomplete information that is seldom authentic. Earlier descriptions of glass furnaces, such as the ones by Vanocchio Biringuccio, Peder Månsson, and Georg Agricola, probably take more of their information from other sources than from the writers' own experience. The scholastic tradition of trusting older authorities more than one's own eyes, combined with the notion of keeping recent accomplishments as secret as possible, makes written sources seldom as fruitful as they may appear at first sight. The authenticity of images must be questioned even more. Did the designer work from actual personal experience, or did he rely on descriptions, other images, or old impressions? Did he accurately depict what he saw, or did he interpret and modify certain details at his own discretion to achieve what he thought was the principal idea?

With the development of glass furnaces from the Middle Ages to the 19th century in view, it seems that a distillation of the large quantity of written and pictorial sources results in surprisingly little evidence. Robert J. Charleston basically distinguishes a "southern," multiple-story-type furnace from the "northern," horizontal arrangement, while Heinrich Maurach points out only two improvements: the development of an altogether different type of furnace after the introduction of coal firing in England in the early 17th century, and the addition of a grate in wood-fired furnaces. As a reference to the latter, Maurach uses an image from Kunckel's Ars Vitraria that does not show an ordinary glass furnace, but rather an experimental furnace for very small glass samples.4 Kunckel himself distinguishes between German, Dutch, and Venetian types of furnaces, but unfortunately, he provides no further details.5 Finally, Rolf-Jürgen Gleitsmann's description of three types of central European glass furnaces seems substantial, although it concentrates on the 18th century: a simple beehive-shaped furnace for forest glass, a more elaborate furnace with controllable air drafts for crystal glass, and a more developed furnace for mirror glass, with a grate for the fire and separation of combustion and melting chambers.6 Gleitsmann subdivides the mirror furnace into five variants, and he lists comparative data, the precision of which is somewhat hard to believe, given the nature of the sources.

Despite all their flaws, written and pictorial documents remain an essential source for the history of the glass furnace. This article is intended to widen the scope of research in terms of both time and subject. Considerable information can be hidden in treatises that address mining rather than glassmaking. The Cadmiologia of the royal Prussian mining director Johann Gottlob Lehmann (Langhennersdorf, Saxony, 1719–St. Petersburg, Russia, 1767) is such a discourse. Its two volumes, published in 1761 and 1766, "sincerely research the history of cobalt" in response to a written query from the recently founded Society for the Encouragement of Arts, Manufactures and Commerce in England.7 Lehmann's work was accepted. He won a prize and became a member of the society.8 From today's point of view, the title Cadmiologia is misleading, since it has nothing to do with the element cadmium, which was discovered by Friedrich Strohmeyer in 1817. Long before that, the term cadmio apparently had various meanings, and Lehmann refers to Georg Agricola, whom he believes to have used cadmiae fossilis metallicae as a term for cobalt.9 In the 1750s, Lehmann had established a factory for manufacturing blue pigment in Hasserode in the Harz Mountains of central Germany. He describes in detail the extraction of the cobalt from the ore, particularly the separation of the poisonous arsenic, in a long horizontal furnace structure. The result of this calcination was essentially cobalt oxide (CoO), which Lehmann called Kobold, but it is better known by the Italian term zaffera.10 To obtain a range of blue colors from the intense black zaffer, and to make it applicable as an enamel color, it was melted into a glass, the "smalt" (which Lehmann called Schmalte, or just Glas). Smalt was a common colorant for various purposes. It was used, for example, as a pigment for paintings and as an enamel for ceramics and glass.11

Fig. 1: Work at a glass furnace. Detail from Kunckel [note 4], plate E after page 62.Smalt itself is nothing more than a glass that contains cobalt—and its manufacture requires a glass-melting furnace. Lehmann's description of this device and the melting process of smalt remains, to my knowledge, the most detailed account of a preindustrial wood-fired glass furnace. However, Lehmann remarks at the beginning of his account that his furnace "looks, for the most part, identical to a common glass furnace," that is, it is similar but not the same.12 Since the purpose of Lehmann's furnace is to melt smalt and scoop it out in portions, but not to permit gathering, it can be assumed that the differences are limited to attachments that facilitate the use of the blowpipe at ordinary glass furnaces, such as hooks, and separate holes that were used to warm up the pipes.13 Lehmann's furnace combines features that Gleitsmann had noted as distinctive characteristics of a mirror glass furnace (the grate and separation of combustion and melting chambers) and a white glass or crystal furnace (the air drafts). To some extent, his furnace shows a rather more complex construction than what is known from other sources. The combustion chamber is divided into three channels: an air draft at the bottom, the ashpit, and the stokehole with the grate. The latter two are connected with each other and with the melting chamber, and it must be assumed that this hole runs through to the air draft, although there is no such indication either in Lehmann's text or in the images, which were engraved by Johann Ernst Gericke in Berlin.14 On the other hand, the lack of smoke outlets seems to be a major shortcoming of this furnace type. The combustion air and smoke are apparently expected to exit the furnace by the working holes, in the same way as shown in an engraving in Kunckel's Ars Vitraria (Fig. 1).15 At that time, flues were a common feature of English coal-fired furnaces, and the properties and advantages of smoke outlets must have been widely known. The fact that, 70 years after Kunckel, the Lehmann furnace still comes without chimneys may therefore be interpreted as a deliberate choice rather than technological backwardness.

Lehmann published the second volume of his treatise in 1766, after he had moved to Russia at the invitation of the Imperial Academy of Sciences. His description coincides with a turning point in the history of glass. On the one hand, glass did not play a significant role in mid-18th century decorative arts. Its production must have been quite stagnant, and all popular attention was directed toward porcelain. On the other hand, technology was on its way to becoming an exact science, and glassmaking in particular evoked scientific interest. In 1760, Paul Bosc d'Antic (1726–1784), a medical doctor who later founded a mirror manufactory at Rouelles, Burgundy, submitted a paper to the French Royal Academy of Sciences on the current state of glassmaking in Europe and on ways to improve it in France.16 His suggestions triggered a revival of the industry. Glassmaking in France had lagged, but within about a generation, that nation had one of the leading glass industries in Europe.17 Bosc d'Antic was not the first to set a new standard. He refers to the writings of the mineralogist Johann Heinrich Pott, who had addressed specific problems of glassmaking, such as the properties of manganese, the nature of gall (impurities that rise to the surface of molten glass), and the making of crucibles in publications of the Royal Academy of Sciences, Berlin, in the 1740s and 1750s. Originally published in French and Latin, his writings were reprinted in German some 30 to 40 years later, possibly to make the contents more accessible to local glass technicians.18 In the following decades, glassmaking received more and more technological attention, beginning with the scrupulous descriptions in encyclopedias such as those by Diderot and d'Alembert (1751–1772), Krünitz (1788), and Jacobsson and Rosenthal (1781–1795).19

Lehmann's description of a glass furnace is extraordinarily detailed. While the precision of his report seems to reflect a new, "technological" age, the furnace itself does not seem to be particularly modern. (As noted above, Lehmann himself regards it as similar to an ordinary glass furnace.) There is some reason to believe that this furnace reflects a technology that was known about two generations earlier, from the end of the 17th century. The development of Baroque crystal at that time probably prompted, or paralleled, some improvements in the related furnaces. Colin Brain has suggested that this development originated in the Low Countries, specifically in the experiments of the German alchemist Johann Rudolph Glauber at the Rozengracht glasshouse in Amsterdam.20 Nevertheless, the thick-walled Baroque crystal, which was almost free of bubbles and impurities, required controllable melting conditions. After that accomplishment had spread throughout central Europe in the early 18th century, not much improvement is visible in the field of hot-working glass for the rest of the century. There was some substantial success in such new decorative techniques as Zwischengoldglas and enameling with transparent colors, but the material itself remained essentially the same.

Paul Bosc d'Antic starts his comments on glassmaking with an anecdote from the mirror glass factory at Saint-Gobain.21 He reports that the director of that company had decided in 1751 to enlarge the size of the crucibles in order to increase the number of mirror casts per melt from three to four. However, the result was disastrous: the melting process took twice as long as before, and the quality of the casting deteriorated dramatically. This would have ruined the company if Bosc d' Antic had not been asked for help in 1754. He soon discovered that an imbalance in the dimensions of the crucibles and the furnace was the crucial mistake. The remedy was to return the crucibles to their original size. Whether this anecdote is altogether trustworthy or not, it shows quite convincingly that any minor change to a glass furnace could have a major and unforeseen impact on production. Glassmakers who succeeded in building a furnace that met their needs probably would not have experimented with its construction. Such changes were confined to the rare moments in which a glassmaker possessed the skills, knowledge, and funds to try something new. This was the case, for example, with Johann Kunckel (d. 1703) in Brandenburg, whose knowledge encompassed chemistry, mining, and glassmaking, and who received apparently unlimited funds from his patron, the great Elector Frederick William of Brandenburg.22 It is very unlikely, therefore, that there were swift changes in furnace technology during the first three quarters of the 18th century. However, as Rolf-Jürgen Gleitsmann had pointed out earlier, Bosc d' Antic's judgment of his fellow glassmakers also applies to our present knowledge of preindustrial glassmaking: "Les fourneaux & les creusets sont la partie la plus importante de l'art de la verrerie, &, j'ose le dire, la moins connue" (The furnaces and the crucibles are the most important part within the art of glassmaking, and, I dare to say, the least known).23

Table IV/Plate IV [Rakow_1000059947_Tab IV_crop_RGB-apd; Collection of the Rakow Research Library, The Corning Museum of Glass, Corning, New York]

Table V/Plate V [Rakow_1000059947_Tab V_crop_RGB-apd; Collection of the Rakow Research Library, The Corning Museum of Glass, Corning, New York]

Table VI/Plate VI [Rakow_1000059947_Tab VI_crop_RGB-apd; Collection of the Rakow Research Library, The Corning Museum of Glass, Corning, New York]

Table IX/Plate IX [Rakow_1000059947_Tab IX_crop_RGB-apd; Collection of the Rakow Research Library, The Corning Museum of Glass, Corning, New York]

Original German Text

English Translation

Johann Gottlob Lehmann, Cadmiologia, oder Geschichte des Farben=Kobolds ... Nebst Beschreibung derer darzu gehörigen Oefen, Maschinen und Arbeiten ... Teil 1, welcher die Etymologie, Geschichte, Arten, Lagerstätten und den mechanischen Gebrauch des Kobolds in sich enthält, 2 vv., v. 1, Königsberg: Gebhard Ludwig Woltersdorfs Wittwe, 1761.

Vierter Abschnitt.

Von dem mechanischen Gebrauch des Kobolds auf Blaufarb=Werken ... [53]

Allein, gleichwie zur Blauen=Schmalte, ausser dem Kobold, auch Pottasche und Sand gehöret, als müssen wir auch hiervon etwas erwehnen. Was Pottasche sey, und wie sie bereitet werde, ist aller Welt bekannt, und hat Kunkel in [58] seiner Glasmacher=Kunst weitläuftig davon gehandelt. Daher wollen wir nur soviel davon anführen, daß man sich bemühen müsse, solche so rein als wie möglich zu schaffen. Die Pottaschen=Brenner wissen besonders künstlich mit der Pottasche, Sand und Kalk zu vermischen. Beydes ist betrüglich und schädlich; ersteres betrüget bey der Machung des Gemenges, indem man alsdenn nicht so viel Sand zusetzen kan, und doch den beygemischten Sand vor Pottasche hat bezahlen müssen. Letzteres, der Kalk, ist eben so schädlich, denn er benimmt der Pottasche viele Kraft, gibt viele Glasgalle, und schläget bey dem Ausschöpfen um sich herum. Beyde Betrügereyen erkennet man, wenn man etwas Pottasche nimmt, solche in Wasser ganz auflöset und filtriret, da man denn in dem Filtrirpappiere den Kalk oder Sand findet. Hat man nun aus dem ganzen Fasse oben, in der Mitte, und unten eine Probe heraus genommen, solche gemischt, verjüngt und gewogen, so kan man hernach ziemlich genau bestimmen, wieviel Kalk oder Sand unter einem Centner Pottasche sey.

Was den Sand anlanget, so ist kein besserer als Kiesel=Steine und Qvarz=Drusen. Es müssen aber solche rein und ohne allen Spath, auch nicht eisenschüßig seyn. Letzteres erkennet man, wenn man sie glühet, und sie werden gelbröthlich, so ist es ein Zeichen, daß sie eisenschüßig, folglich untauglich sind. Wenn sie aber rein sind, so werden sie im Calcinir=Ofen scharf calcinirt, naß gepocht, damit wenn ja noch etwas sehr weniges Unreines dabey, solches mit der Trübe davon laufe, das Gute aber zurück bleibe. Hierauf wird dieser zart gepochte Sand ausgeschlagen, in einen Calcinir=Ofen geworfen, scharf durchgeglühet, gesiebet, und zum Gebrauch verwahret. Es lieget an dem Sande fast das meiste mit, denn, ist er eisenschüßig, so fällt die Farbe tumm. Ist er mit Spath vermenget, so frist er viel Pottasche, und giebt viel Glasgalle, die Sande aber selbst bleiben als weisse, zarte Stäubgen in dem Glase zum Theil sitzen, und machen eine schlechte Farbe. Ist er thonartig, so flüßt er noch weniger, und alsdann ist die Farbe die schlechteste von der Welt. Wenn nun also alles gehörig vorgerichtet ist, so schreitet man

2) Zum Schmelzen selbst. Hierzu werden nun dreyerley Ofens erfordert, als 1) der Schmelz=Ofen selbst. 2) Der Temper=Ofen. 3) Der Darr=Ofen. Was den ersten Ofen, nehmlich den Schmelz=Ofen anlangt; so zeiget dessen ganze Gestalt Tab. IV. Er siehet einen [59] gemeinen Glaß=Ofen großtentheils gleich. Die Structur desselben ist folgende, wenn solcher, wie am gewöhnlichsten, zu 6. Häfens eingerichtet warden soll. Erstlich wird darauf gesehen, daß unten ein fester, trockner Boden sey, dieser wird 12. Fuß lang und 11. Fuß breit eben gemacht. Hierauf wird ein Abzug übers Kreutz, wie Tab. V. Fig. 1. zeiget, von festen Steinen gemauert, 1. Fuß tief und 1. Fuß breit. Diese 4. Abzüge müssen jeder auf seiner Seite, aus den Mittelpunct abschüßig liegen, damit kein Wasser darinnen stehen bleiben könne. Oben werden diese Abzüge mit festen, breiten Steinen zugedecket. Wenn diese Canäle fertig, so werden die 4. Quartiere, a. b. c. d. welche zwischen diesen Canälen liegen, mit guten festen Steinen ausgesetzet und feste gemauert, so daß der ganze Umfang eine Wasserrechte Ebne vorstelle. Auf dieser Ebne wird hernach recht mitten, der Länge des Ofens nach 2. und ein hal ben Fuß weit, 2. Fuß hoch, unter dem ganzen Ofen der Länge nach weg, das Aschenloch angeleget, wie solches Tab. VI. c) im Grundriß zu sehen. Wenn dieses Aschenloch die Höhe von 2. Fuß mit seinen Seitenmauern erreichet hat, so wird es mit einen flachen Gewölbe gedeckt, und zwar von beyden Seiten, ohngefehr von 4. und ein halben Fuß lang, die übrigen 3. Fuß bleiben offen. Alsdann wird Tab. V. Fig. 2. der Heerd zum Schürloche gemacht, und über den ofnen Felde, Tab. V. Fig. 2. Tab. VI. d. e. zugleich der Rost*) von festen Mauersteinen gesetzet, auf diesen Heerd kommt nachgehends das Feuer zu liegen. Der Rost und dieser Heerd müssen von gleicher Höhe seyn. Auf diesen Heerde werden die Seitenmauren 1. und ein halben Fuß hoch und eben so weit zum Schürloche angelegt, deren eines forne, das andre hinten gegen einander über sind. Die Mauern neben den Roste müssen mit denen Mauern in dem Schürloche oben in einer Gleiche zu stehen kommen, doch daß sie etwas überhängend gemauert werden. Wenn nun also die Mauren in dem Schürloche ihre Höhe erreichet haben, wird oben her, wie in den untersten Gewölbe flach gewölbet, nemlich einen halben Fuß stark. Die Oefnung mitten im Gewölbe bleibt 2. Fuß lang, 1. und einen halben Fuß weit offen. Auf dieses Gewölbe kommt der Heerd, auf welchen die Häfens zu stehen kommen, oder das Gefäße, Tab. V. Fig. 3. 4. und Tab. VI. f. dieser bestehet aus Ziegelsteinen, welche aber sehr gut seyn müssen, am besten, wenn solche von dem Meister selbst aus reinen Leimen, zerstoßnen alten Häfen, und sehr [60] wenig feinen Sande gemacht werden, so, wie andre Backsteine geformt und erst gelinde an der Luft getrocknet, hernach aber tüchtig gebrennt worden. Dieser Heerd muß recht eben seyn, und in der Mitten gehet die Oefnung Tab. VI. g. durch, durch welche das Feuer spielet, welche 2. Fuß lang und 1. und einen halben Fuß weit ist. Auf diesen Heerd werden alsdann die Häfen gesetztet, deren hier Tab. V. Fig. 4.6. a.b.c.d.e.f. und Tab. VI. dreye h.i.k. angezeiget werden. Wenn nun also das Gefäße Tab. V. Fig. 4. fertig ist, so wird der Kranz, Tab. VI. 1. zum Ofen angelegt, dieses ist eine runde Einfassung des Gefäßes, welche wenigstens 1. und einen halben Fuß stark von keilförmigen, aus guten Feuerhaltenden Thon bestehenden Steinen gemacht wird, seine Höhe ist 3. und einen halben Fuß. Ganz unten kommt das Werkloch, Tab. IV. b. dessen Boden ganz horizontal mit dem Gefäße seyn muß, die Höhe aber desselben ist 2. Zoll höher und weiter, als die hinein zu setzenden Häfen sind; durch dieses Loch werden die getemperten Häfen eingesetzet, b.b.b. hingegen sind m.m.m. Tab. VI. und Tab. IV. d. die Schöpf=Löcher, durch welche das Gemenge zum Glase eingeleget, und das geschmelzte Glas ausgeschöpfet wird, diese sind 14. Zoll hoch, oben rund, und 7. Zoll weit. Unter diesen Schöpflöchern befinden sich die Stichlöcher, welche 6. Zoll weit und 8. Zoll hoch, und deren unterstes ebenfalls mit dem Gefäße wagerecht seyn muß. c.c. Tab. IV. Wenn nun diese Kranzmauer obbesagte Größe erreichet hat, so werden wiederum die Sohlen zu der Haube n. Tab. VI. und e. Tab. IV. von obbesagten keulförmigen Rinksteinen gelegt, und alsdann ein sehr starker eiserner Reifen, f. Tab. IV. und g. Tab. IV. welcher scharf angezogen warden muß, herum geleget, damit die Hitze den Ofen nicht auseinander treiben könne. Auf diese Sohle wird alsdenn die Haube wie bey andern Glas=Oefen von keilförmigen Steinen 3. Fuß hoch als ein Gewölbe gesetzt, und wohl mit denen, das ganze Gewölbe zusammen haltenden Pfeilern f. Tab. IV. verbunden. Wenn nun auf diese Art der Ofen fertig ist, so wird solcher innwendig, wo die Häfens zu stehen kommen, mit einem guten dem stärksten Feuer widerstehenden luto ausgeschmiert, welches auch auswendig geschehen muß. Nun lässet man den Ofen gehörig austrocknen.

Was nun zweytens den Temper=Ofen anlangt, so ist an solchen nichts besonders, sondern er ist eben so gebauet, wie die bey allen Glashütten gewöhnlichen Temper=Ofens. [61]

Der dritte, als der Darr=Ofen, wird bloß darzu gebraucht, urn das Holz darinnen zu trocknen. Dieser Ofen, welcher gewölbet ist, und 6. Fuß lang, und 6. Fuß weit ist, bekommt seine Hitze von dem Glas=Ofen, an welchen er gebauet; in solchen werden die Scheidholze zum Schüren, nachdem sie gespalten und kleine gemacht, aufgeschichtet, damit sie recht dürre werden; doch muß die Hitze so temperirt seyn, daß es zwar schwarz anlaufe, aber nicht sich entzünde.

Nun ist noch übrig die Glas=Häfen selbst zu machen. Hierzu wird ein reiner, von Sand, Qvarz, Spath, und metallischen Theilen, ganz befreiter Thon erfordert, dieser wird sauber geschlämmt, und mit klar gepochten alten Häfen vermischt. Gemeiniglich nimmt man zu zwey Theilen frischen Thon, ein Theil von alten Häfen. Ich habe gefunden, daß es nicht so gut ist, wenn, wie in der Obersächsischen Berg-Akademie gesagt wird, alte abgesetzte Häfen darzu genommen werden; weil in solchem Falle, das noch darinne befindliche Glas Ieicht in Fluß geräth, ja man hat es auch nicht einmal nöthig, denn es giebt ja bey denen Häfens bestandig Abgang, bald zerspringt einer bey dem Trocknen an der Luft, bald reißt einer bey dem Tempern, bald aber geht einer bey dem Ausheben aus dem Temper=Ofen und einsetzen in den Glas=Ofen entzwey, so, daß man leider öfters schadhafte Häfens genug hat, und daher wohl thut, wenn man lieber einen Hafen zum Ueberfluß mehr tempert, urn damit den Platz des entzwey gegangenen ersetzen zu können. Wenn nun also die Mischung vom frischen und alten Thone gehörig gemacht, und solcher Thon recht zähe ist, sowerden die Häfen folgender Gestalt geschlagen: Erstlich nimmt der Farbmeister, welcher hierbey gemeiniglich die Stelle eines Töpfers vertrit, den angefeuchteten zähen Thon, schlägt solchen tüchtig und derb, und schneider daraus den Boden, welcher 3. Zoll dicke seyn muß, diesen legt er zum Grunde. Hierauf hat er eine Forme von der Grösse, wie die Häfen seyn sollen, welche aus Faßtauben in einander gefüget, und feste ist, dem Ansehen nach, wie ein Faß ohne Boden 2. Fuß hoch, und oben 2. Fuß weit, inwendig aber hohl; diese Forme setzt er auf den ausgeschnitrenen Boden von Thon, und urn diese Forme schläget er eine nasse, dichte Leinewand, schneidet alsdann von dem geschlagenen Thone ein grosses Stücke, welches just die Höhe und Weite des zumachenden Hafens giebt, legt solches urn die Forme über die darum geschlagene Leinewand, schlägt es feste an, verbindet es mit dem Boden, und ver= [62] schmiert dessen Fugen, daß es also eine umgekehrte, abgekürzte, conische Figur gibt, verbindet es mit den untern Boden so feste als möglich, läßt es eine Weile stehen, daß es windtrocken wird, ziehet alsdann die Forme samt der Leinewand heraus, und läßt den Hafen ganz trocken werden, und ist der Rand alsdann 2. Zoll stark. Diese Häfen werden alsdann an einem trocknen Orte, damit sie keine Feuchtigkeit anziehen können, bis zum Gebrauch verwahret. Wann dergleichen Häfen gut gemacht sind, so stehen sie wohl ein halbes Jahr gut im Feuer. Nun gehen die Arbeiten zum Schmelzen selbst an.

Wenn nun also alles vorgerichtet ist, so wird der Glas=Ofen einige Tage lang angewärmet, das ist, man macht Feuer hinein, daß der Ofen nach und nach warm und heiß werde, und endlich den Grad der Gluth erreiche, der zum Glasschmelzen erfordert wird. Man thut wohl, wenn man hierzu 3. bis 4. Tage, ja noch Iänger Zeit läßt, damit der Ofen nach und nach erglühe, sonst pflegt er leicht Risse zu bekommen. Indessen heitzet man auch den Temper=Ofen, welcher in der Hütte, und nahe bey dem Glas=Ofen seyn muß; setzet die Häfen ein, daß sie trocken, heiß und glühend werden. Die Schöpflöcher sind mit eisernen Vorsetzen zugesetzt, deren Gestalt Tab. 4. h. angezeiget wird: Sie sind von geschmiedeten Eisen 1. Zoll stark, und 1. Zoll höher und breiter als die Schöpflöcher. Der obere Ausschnitt daran ist, daß die Luft hinein spielen könne; das Loch in der Mitten aber, damit man sie mit einen eisernen Haken abheben und wieder vorsetzen könne. Die darunter befindlichen Stichlöcher sind indessen gleichfals zugemauert, das Werkloch aber feste zugesetzet. Wenn nun der Ofen in seiner gehörigen Hitze, und die Häfen sattsam getempert, so wird das Werkloch aufgemacht, die heißen Häfen auf eisernen Stangen hinein gebracht, in Ordnung gestellt, hernach wird dieses Loch vermauert; wobey zu mer ken, daß ein Hafen von den andern nur 2. Zoll abstehen darf, doch muß die mittelste Oefnung frey bleiben, damit das Feuer durchspielen kan. Nachdem sie nun in voller Gluth sind, so wird oben zu denen 6. Schöpflöchern, deren jedes just über einen Hafen stehet, das erste Gemenge eingeleget. Besser aber ist, wenn man Sumpfeschel hat, daß man solche das erste mal einlege, damit sich die Häfen innwendig damit verglasen. Was Sumpfeschel sey, werden wir besser unten hüren. Da es aber gewiß, daß je länger der Ofen gehet, je mehr kommt er in die Hitze, so ist nicht zu bestimmen, wie lange man das [63] Glas müsse stehen lassen, ehe manes ausschöpft. Gemeiniglich schöpfet man die ersten 6. bis 8. Tage aller 12. Stunden. Allein, hier ist durchgehends keine Zeit fest zu setzen, sondern der Farbmeister probiret so lange mit dem Rühreisen, bis er siehet, daß das Glas reine geflossen. [zu den Eigenschaften des Kobalt-Gemenges] [64]

Wenn nun der Farb=Meister alles dieses genau weiß und kennet, so wäget er von Kobold, und was zu demselben kommen soll, als Speise, Heerdglas, Sumpfeschel, ferner von Sand und Pottasche so viel ab, als er zu einen Gemenge in die 6. Häfen nöthig hat, läßt alles in den bey dem Glas=Ofen stehenden Gemeng=Kasten stürzen, mit Schaufeln wohl und fleißig durch einander mischen, und wenn es sorgfältig gemischt, mit eisernen Eintrage-Schaufeln in die glühenden Häfen einlegen. Diese Schaufeln sind von eisernen Blech, wie Kohl=Schaufeln, 1. Fuß lang, 6. Zoll weit, Tab. IX. d) und eben so viel tief, mit einen langen, starken, eisernen Stiel. Mit denenselben wird das Gemenge in die Häfen eingelegt, hierauf die Schöpflöcher mit denen eisernen Vorsätzen Tab. IV. h) zugesetzt, und scharf zugefeuert. Wenn man denn siehet, daß das Glas schmelzet, welches bald eher, bald später geschiehet, je nachdem die Kobolde und Sande strenge oder leichtflüßig sind, so wird das Glas in denen Häfen mit starken Rühreisen, welche vorher warm gemacht worden, umgerührt, und damit aile halbe Stunden fortgefahren, bis man aus dem an denen Rühreisen hangen bleibenden Glase gewahr wird, daß es gut geflossen, welches man daran erkennet, wenn das Glas durch und durch gleiche tingirt, und keine weiße Körngen, als Zeichen des noch nicht genug geflossenen Sandes, mehr darinne zu sehen sind. Hierauf wird das Glas mit starken geschmiedeten eisernen Löffeln, Tab. IX. b) welche 8. Zoll in Diameter und 3. Zoll tief sind, ausgeschöpfet, und in ein ganz nahe dabey gestelltes grosses Faß gestürzet. Dieses Faß muß wenigstens 4. Fuß hoch, und eben so viel in Diameter groß seyn, welches voll kalt Wasser, und pflegt so gestellt zu werden, daß beständig kalt Wasser durch eine Röhre in solches zulaufe, um das hinein gestürzte Glas, desto eher abzukühlen. Weil aber [65] besonders bey speisigen Kobolden, in Glasschmelzen viele Speise entstehet, nicht weniger auch öfters sich Glasgalle findet, die beyderseits, wenn sie in das kalte Wasser kommen, mit grossen Knall und Gefahr derer Arbeiter um sich schlagen, so hat man eiserne Pfannen, Tab. IX. c) in welche man, ehe man das Glas in das Wasser wirft, die Speise und Glasgalle aus den Löffel laufen läßt. Ich muß hierbey etwas erinnern: Die Mineralogen behaupten, es vermische sich der Wißmuth niemals mit der Koboldspeise, es ist aber dieses nicht so gar richtig, wovon zu anderer Zeit ein mehreres. Nachdem nun also die Häfen reine ausgeschöpft, wird ein frisches Gemenge eingelegt, und damit so lange fortgefahren, als man es nöthig befindet, oder die Häfen halten, dieses ist, wie gesagt, willkührlich. [66]

Johann Gottlob Lehmann, Cadmiologia, or History of the Color Kobold24 ... with a Description of the Associated Furnaces, Machines, and Works ... Part 1, Which Contains the Etymology, History, Types, Ore-Deposits, and Mechanical Use of Cobalt, 2 vv., v. 1, Königsberg: Gebhard Ludwig Woltersdorfs Wittwe, 1761.

Fourth section. Of the mechanical use of cobalt in blue-coloring factories ... [53]25

Since apart from cobalt, potash and sand also belong in the blue-colored smalt, we have to say something about these, too. Everybody knows what potash is and how to prepare it, and Kunckel [58] has dealt with it extensively in his Art of Glassmaking.26 Therefore, we would only like to add that one should try to produce it as pure as possible. The potash burners know how to mix sand and lime diligently into the potash. Both [additives] are fraudulent and harmful. The first [sand] is deceiving when mixing the batch because one cannot add as much sand [as planned], having already paid for the sand [the price of] potash. The latter, lime, is as harmful because it reduces the strength of the potash, produces lots of glass gall, and splatters when gathering. Both frauds can be detected by taking some of the potash, dissolving it in water, and filtering it, since one will find the lime or sand on the filter paper. Once samples have been taken from the top, middle, and bottom part of a whole barrel, mixed, reduced, and weighed, the amount of lime or sand within a hundredweight27 of the potash can be determined fairly precisely.

As for the sand, there is nothing better than quartz pebbles and quartz ore. However, they have to be pure, without any spar,28 and not interspersed with iron. This can be detected if they become yellow-red when heated and glowing. This is a sign that they contain iron and are therefore useless. If, however, they are pure, they should be calcined sharply in a calcar, moistened, and pounded.29 This should make even the slightest impurity that may have been left, be washed away with the cloudiness [of the water], while the good material is left behind. Next, this soft powdered sand will be beaten out [i.e., freed from the water] and thrown into a calcar, strongly heated, sifted, and kept in storage for further use. Almost everything depends on the sand because, if it is interspersed with iron, the color will turn dim. If it is blended with spar, it will consume much potash and return much gall, while the sand itself partly resides as a white, soft dust within the glass, thereby making a bad color. If [the sand] is claylike, it flows even less, and the color becomes the worst on earth. Now if everything is properly prepared, one moves to

2. The melting itself. Three types of furnaces are necessary for this purpose: (1) the melting oven itself, (2) the lehr [annealer], and (3) the drying kiln. As for the first, the melting furnace, plate IV shows it in its entirety. It looks, [59] for the most part, identical to a common glass furnace. Its structure is as follows if, as is normally the case, the furnace is intended to hold 6 pots. First, one has to make sure that the ground is solid and dry. It must be leveled to 12 feet long and 11 feet wide.30 On this, a cross-shaped draft is built of solid stones, as shown in plate V, fig. 1, one foot deep and one foot wide. Each of these four air holes has to slope down from the center to the rim in order to avoid standing water. Above, the drafts are covered with solid, broad stones. Once these channels are completed, the four quarters between them (a, b, c, d) are solidly paved with good, solid stones, so that the whole circumference becomes a horizontal plane. On this plane, the ash pit is laid out, crossing the center from one side to the other, 2 1/2 feet wide and 2 feet high, as is shown on the ground plan, plate VI, c. Once the side walls of this ashpit reach their height of two feet, they are covered with a flat vault, that is, from both sides about 4 1/2 feet long. The remaining 3 feet [in the middle] are left open. Thereupon, as shown in plate V, fig. 2, the fireplace for the stokehole may be built. Also (plate V, fig. 2, and plate VI, d, e), the grate ([plate V, fig. 2,]*) above the open field [of the ashpit] is made out of solid bricks. This will later be the place for the fire. This fireplace and the grate must have the same height. The side walls of this fireplace are set up 1 1/2 feet high and as wide, forming the stokeholes, one of which is at the front, the other at the back of the furnace, directly opposite each other. The walls [of the ashpit] next to the grate and the walls of the stokehole must be exactly on top of each other [literally, in an evenness] but built in a slightly sloping manner [literally, slightly overhanging]. Now if the walls of the stokehole have reached their height, they should be covered with a flat vault, like the vault below, that is, 1/2 foot thick. The gap in the middle of the vault stays open 2 feet long and 1 1/2 feet wide. On this vault is built the hearth on which the crucibles, or pots, will stand (plate V, figs. 3, 4; plate VI, f). This place is made out of bricks that have to be very good. They are best when made by the master himself out of pure clay,31 crushed old crucibles, and very [60] little fine sand. They need to be formed like ordinary bricks, first dried lightly in fresh air, but afterward fired thoroughly. This fireplace must be quite even, and the opening, through which the fire blazes up, runs through the center (plate VI, g), 2 feet long and 1 1/2 feet wide. On this level are set the crucibles (six in plate V, fig. 4, a, b, c, d, e, f; three in plate VI, h, i, k). Once this receptacle (plate V, fig. 4) is completed, the cornice of the furnace is begun (plate VI, 1). This is a ring-shaped wall around the receptacle that has to be at least 1 1/2 feet thick and built of wedge-shaped stones that are made out of good clay that can withstand the fire. Its height is 3 1/2 feet. At its bottom is the access hole [literally, manhole] (plate IV, b). Its surface level has to be even with the pots, but its height is 2 inches taller and wider than the crucibles that are to be put in. Through this opening, the tempered crucibles are introduced ([plate IV,] b, b, b), while plate VI, m, m, m, and plate IV, d, show the working holes, through which the glass batch is put into the pots and the molten glass is collected. These are 14 inches high, round on the top [vaulted], and 7 inches wide. Below these working holes, there are the foot holes [literally, sting holes], which are 6 inches wide and 8 inches high.32 The ground level of each has to be even with the pot (plate IV, c, c). Now if the cornice of the furnace has reached the abovementioned height, the groundsill for the vault (plate VI, n, and plate IV, e) is laid out again with the abovementioned wedge-shaped ring stones. A very strong iron band (plate IV, f, g), which has to be tightened firmly, is then laid around to prevent the heat from shattering the furnace. Thereupon, the top is built on this groundsill, using wedge-shaped stones, as in other glass furnaces. It is built 3 feet high as a vault and connected to the buttresses (plate IV, f) that hold the whole vault together. Now if the furnace is set in this way, it must be covered inside, where the crucibles are going to stand, with a sound plaster that can withstand the strongest fire. This also has to be done on the outside. Now the furnace has to be left to dry properly.

Secondly, as to the lehr, there is nothing special about it. It is built in the same way as the usual lehrs in all glasshouses.33 [61]

The third [type of furnace], that is, the drying kiln, is used merely to dry wood. This furnace is vaulted, 6 feet long and 6 feet wide, and receives its heat from the glass furnace, adjacent to which it is built. In it, the wood logs for stoking are piled up, after they have been chopped into small pieces, to let them dry thoroughly. However, the heat must be regulated to allow them to turn black but not to catch fire.

Now, all that is left is to make the crucibles themselves. Hereto, a pure clay is necessary, which is free of sand, quartz, spar, and metallic parts. It must be washed clean and mixed with finely crushed old crucibles. Usually, one part of old crucible material is added to two parts of fresh clay. I have found that it is not recommended to use old crucibles with [glass] deposits, as is advised in the Upper Saxon Mining University,34 because the glass easily melts. This [use] is not even necessary because there is continuous waste among crucibles: now one shatters while being dried in the air, then one is torn apart during firing, then one shatters when taken out of the kiln or put into the glass furnace. So, unfortunately, there often are enough damaged crucibles, and one does well to fire one extra pot in order to replace a damaged one. So, as soon as fresh and aged clay has been mixed properly, and such clay becomes leathery, the crucibles are made [literally, beaten] as follows: First, the color master,35 who in this case usually acts in the place of a potter, takes the leathery, moistened clay, kneads [literally, beats] it hard and thoroughly, and cuts the bottom from it, that has to be 3 inches thick, and starts with this basis. Then he takes a mold, in the size of the future crucibles, that is solid and made out of cask staves that are fitted into one another, like a barrel without a bottom, 2 feet high and, at the top, 2 feet wide, but hollow on the inside. He sets this mold onto the cut-out bottom of clay, and covers the form with moistened, closely woven canvas. Thereupon he cuts a large slice from the beaten clay, of just the height and width as the future crucible, and wraps it around the mold and the canvas with which it is covered. He fits it tightly, joins it with the bottom, and fills in the gaps, so that it gains an inverted, truncated conical shape. He joins it with the lower bottom as tightly as possible and lets it rest for a while to become air-dried. Subsequently, he pulls out the mold and the canvas and lets the crucible dry completely, after which the rim is 2 inches thick. Afterward, the crucibles will be stored in a dry place, where they cannot attract humidity, until they are needed. If such crucibles are made properly, they can endure well in the fire for about half a year. Now, the work for melting begins.

Once everything is prepared, the glass furnace is warmed up for a couple of days, that is, one sets a fire to warm and heat up the furnace little by little, until it finally reaches the glow that is needed to melt glass. It is recommended to take 3 to 4 days, or even longer, for this, to allow the furnace to heat up [literally, glow] because it otherwise tends to crack easily. Meanwhile, the lehr is also lit, which has to be located within the glasshouse, next to the glass furnace. The crucibles are set [into the lehr] to make them dry, hot, and glowing. The working holes [of the glass furnace] are closed with iron covers, the shape of which is shown in plate IV, h. They are made out of wrought iron, 1 inch thick, and 1 inch higher and wider than the working holes. Their top cutout allows air to flow in; the hole in the center, however, allows them to be removed and applied with an iron hook. The foot holes below are also walled up, and the opening through which the pots are introduced is tightly closed. Once the furnace has reached the right heat, and the crucibles are sufficiently fired, the access hole is opened, the hot crucibles are introduced on iron bars and set into the right order, and the opening is bricked up; yet it has to be noted that one crucible should stand only 2 inches apart from the next one, and that the central opening must be cleared, to allow the fire to blaze up. After they have reached their full glow, the first batch is introduced through the working holes, each of which is located above one of the crucibles. However, it is better to first introduce a powder blue [Sumpfeschel], in order to line the inside of the crucibles. The meaning of Sumpfeschel is better explained below.36 Since it is certain that a furnace has more heat the longer it runs, it cannot be determined how long [63] the glass has to stay before it is collected. Usually, it is scooped every 12 hours during the first 6 to 8 days. Yet, the time cannot be determined; instead, the color master has to test it with a stirring pole until he finds that the glass has melted purely. [A discussion of the properties of the cobalt batch follows.] [64]

Once the color master knows all this, he weighs the cobalt and what is supposed to be added, like speiss, hearth glass, powder blue, and as much sand and potash as is needed or a batch sufficient for 6 crucibles, and lets it all be thrown into a box next to the furnace.37 It is mixed thoroughly and diligently with shovels, and once it is carefully mixed, it is introduced into the glowing crucibles with iron shovels. These shovels are made out of sheet iron, like coal shovels, 1 foot long, 6 inches wide and as deep, with a long, strong iron handle (plate IX, d). With these, the batch is introduced into the crucibles, and then the working holes are closed with the iron covers (plate IV, h), and the furnace is strongly lit. As soon as the glass is seen to melt, which can happen earlier or later, depending on whether the cobalt and sands are refractory [literally, stern] or easily fusible, the glass in the crucibles is stirred with strong stirring irons that have been preheated. This is continued every half-hour, until it can be seen from the glass that sticks onto the iron rod, that it has melted properly. This is the case when the glass has one color throughout, and no white grains are noticeable anymore as a sign of unmelted sand. Thereupon, the glass is collected with strong wrought-iron scoops (plate IX, b) that are 8 inches in diameter and 3 inches deep, and thrown into a big barrel very close to the furnace. This barrel has to be at least 4 feet high, and as wide. It is filled with cold water and usually positioned in such a way that a continuous stream of cold water runs into it through a tube, to cool down all the faster the glass that has been thrown into it. But because, [65] especially with impure cobalt, lots of speiss remains in the glass melts, and no less gall can be found, and both, if they touch the cold water, would splatter with a loud bang and put the workers in danger, there are iron pans (plate IX, c), into which the speiss and gall are poured from the scoop before the glass is thrown into the water. I must offer a reminder here that the mineralogists claim that bismuth would never mix with the cobalt speiss. However, this is not fully correct, as will be discussed later. After the crucibles have been cleared out entirely, a fresh batch is introduced, and this should be continued as long as it is considered necessary, or as the crucibles last, which is, as has been noted, indeterminate. [66]

Dedo von Kerssenbrock-Krosigk
This article was published in the Journal of Glass Studies, Vol. 47 (2005), 121–136.

1. Gottfried Wehen, report on a visit to the glasshouse of Walkersbach on December 14, 1649, reprinted in Walter Lang and others, Spätmittelalterliche Glasproduktion im Nassachtal, Uhingen, Kreis Göppingen, Materialhefte zur Archäologie in Baden-Württemberg, v. 59, Stuttgart: Theiss, 2001, pp. 177–178: "Es hatt mir der Obman Jörg Greiner gesagt, das der Schmöltz Off 18 oder 20 Wochen kenne gebraucht werden alsdan mieße man ihne wider neyw setzen" (The master Jörg Greiner told me that the melting furnace would last about 18 to 20 weeks, and that it must be put up anew thereafter).

2. Heinrich Maurach, "Glasschmelzöfen in alter Zeit," Glastechnische Berichte, v. 12, no. 8, 1934, pp. 265-274; Robert J. Charleston, "Glass Furnaces through the Ages," Journal of Glass Studies, v. 20, 1978, pp. 9–25.

3. Heinz Horat, "Les Fours de verre dans des traités technologiques: Problèmes d'interprétation d'après l'exemple de Biringuccio," Archeologia e storia della produzione del vetro preindustriale, Atti del Convegno Internationale "L'attività vetraria medievale in Valdelsa ed il problema della produzione preindustriale del vetro: Esperienze a confronto," April 2–4, 1990, ed. Marja Mendera, Florence: Edizioni All'Insegna del Giglio, 1991, pp. 439–450.

4. Charleston [note 2], pp. 32–33; Maurach [note 2], pp. 269–270; Johann Kunckel, Ars Vitraria Experimentalis, Oder Vollkommene Glasmacher-Kunst ..., Frankfurt-Leipzig: Johann Bielcke, 1679, plate M before p. 209. Kunckel's book is a later edition of Neri's Arte Vetraria of 1612, with extensive comments and additions.

5. Kunckel [note 4], comment on chapters 24 and 25, p. 62.

6. Rolf-Jürgen Gleitsmann, "Zur Leistungsfähigkeit von Glasschmelzöfen des 18. und frühen 19. Jahrhunderts. Ein Vergleich nach Angaben aus der technologischen Literatur," Glastechnische Berichte, v. 59, no. 3, 1986, pp. 64–75.

7. Johann Gottlob Lehmann, Cadmiologia, foreword. Three letters by Lehmann from 1757 to 1760 are preserved in the Royal Society's archive.

8. For a biography of Lehmann, see Bruno von Freyberg, Johann Gottlob Lehmann, ein Arzt, Chemiker, Metallur, Bergmann, Mineraloge und grundlegender Geologe, Erlanger Forschungen B, 1, Erlangen: Universitätsbund, 1955; idem, "Lehmann, Johann Gottlob," Dictionary of Scientific Biography, ed. Charles C. Gillispie, v. 7, New York: Simon & Schuster 1981, pp. 146–148.

9. Lehmann [note 7], v. 1, pp. 4–7.

10. Today, a clear distinction is made between zaffera (zaffer) and smalt, but the terms were often synonymous in earlier writings, such as Diderot's Encyclopédie.

11. Cf. Josef Horschik, "Beiträge zur Geschichte der Kobaltfarbe und ihrer Verwendung in der Keramik," Keramos, no. 85, 1979, pp. 119–142; and Heike Stege, "Out of the Blue? Considerations on the Early Use of Smalt as Blue Pigment in European Easel Painting," Zeitsschrift für Kunsttechnologie und Konservierung, v. 18, no. 1, 2004, pp. 121–142.

12. Lehmann [note 7], v. 1, pp. 59–60. Yet Lehmann himself uses the term "glass furnace" (Glas=Ofen) throughout his figure captions.

13. The process of smalt-making had been depicted by August Fürchtegott Winkler in 1790. Cf. Maria and Hans-Heinz Emons, "Blaufarben-Blåfarve": Ein historisches Beispiel deutschnorwegischer Zusammenarbeit, Det Kongelige Norske Videnskabers Selskab. Skrifter 2, ed. Harald Nissen, Trondheim: Tapir Akademisk Forlag, 2000, figs. 4–11.

14. Gericke, who was a portraitist in Berlin, almost certainly engraved according to Lehmann's instructions, and not from personal experience.

15. Kunckel's abovementioned experimental furnace in plate M is equipped with flues.

16. Paul Bosc d'Antic, "Quels sont les moyens les plus propres à porter l'économie & la perfection dans les verreries de France?," in Oeuvres de M. Bosc d'Antic, contenant plusieurs mémoires sur l'art de la verrerie, sur la faiencerie, la poterie, l'art des forges, la minéralogie, l'électricité et sur la médecine, 2 vv., v. 1, Paris: rue et hôtel Serpente, 1780, pp. 50–257.

17. Fernando Montes de Oca, L'Age d'or du verre en France, 1800-1830. Verreries de l'Empire et de la Restauration, Levallois: the author, 2001; cf. the review by Dedo von Kerssenbrock-Krosigk, Kunstchronik, v. 57, nos. 9/10, 2004, pp. 463–467.

18. Johann Heinrich Pott, "Examen chymicum magnesiae vitriariorum, Germanis Braunstein," Miscellanea Berolinensia, v. 6, 1740, pp. 40–53 (republished in Physikalische und medicinische Abhandlungen der Königlichen Academie der Wissenschaften zu Berlin, v. 2, 1781, pp. 244–264); idem, "Recherches sur la nature et les propriétés du fiel de verre," Histoire de l’Académie Royale des Sciences et des Belles-Lettres de Berlin, 1748, pp. 16–27 (republished in Physikalische und medicinische Abhandlungen ..., v. 3, 1783, pp. 315–330); idem, "Essai sur la manière de preparer des vaisseaux plus solides, qui puissent soutenir le feu le plus violent, er qui soient les plus propres à contenir les corps en fusion," Histoire de l'Académie Royale ..., 1750, pp. 98–143 (republished in Physikalische und medicinische Abhandlungen ..., v. 3, 1783, pp. 629–684).

19. Johann Georg Kruenitz, "Glas," in idem, Oekonomisch-technologische Encyklopaedie, v. 18, 1788, pp. 580–680; Johann Karl Gottfried Jacobsson and Gottfried Erich Rosenthal, Technologisches Wörterbuch, 8 vv., Berlin-Stettin: F. Nicolai, 1781–1795. Cf. Günther Stein, "Beschreibung von Glasöfen in 'J.K.G. Jacobssons technologischem Wörterbuch,' 1781–1795," Glastechnische Berichte, v. 42, no. 10, 1969, pp. 432–439.

20. Colin Brain, The Technology of 17th Century Flint Glass, n.d. [2001]. Cf. Peter Francis, "The Development of Lead Glass. The European Connections," Apollo, the International Magazine of the Arts, February 2000, pp. 47–53.

21. Bosc d'Antic [note 16], pp. XVIII–XXV and 164.

22. Dedo von Kerssenbrock-Krosigk, Rubinglas des ausgehenden 17. und des 18. Jahrhunderts, Mainz: Philipp von Zabern, 2001, pp. 36–40.

23. Bosc d'Antic [note 16], p. 67. Cf. Gleitsmann [note 6], pp. 64–65.

24. German Kobold means "goblin" or "imp." According to Lehmann, it is the term from which the name of the mineral cobalt (Co) is derived. Since cobalt usually occurs in combination with arsenic, miners were easily poisoned. The enigmatic source of this poisoning—and of some of the symptoms, such as muscle cramps—may have led to the association of cobalt with a mischievous demon. However, another possible source for the word "cobalt" is Czech kowalty (ore-bearing; cf. Horschik [note 11], p. 131).

25. Bracketed numbers throughout this section refer to the page numbers in Lehmann's original publication. Here, the text preceding the number appears on the stated page.

26. Kunckel [note 4], pp. 347–351 (slightly extended in the second edition of 1689, pp. 316–328). For the history of potash, see Werner Loibl, Asche zu Glas. Die Flußmittel Asche, Pottasche und Soda in fränkische Glashütten vom 17. bis zum 19. Jahrhundert, Schriften zur Glassammlung des Spessartmuseums 2. Schriften des Geschichts- und Museumsvereins Lohr a.M. 29 (exhibition catalog, Lohr, 1996), Lohr: the institution, 1996.

27. 1 Zentner (hundredweight) = 50 kilograms.

28. Calcium fluoride (CaF2).

29. Lehmann ([note 7], v. 1, pp. 56–58 and pls. II and III) provides a description of a calcar, or roasting furnace, for calcining the cobalt ore.

30. The measures of length differed between countries. The sizes in this text should be Prussian (i.e., one inch equals 2.615 centimeters, and 12 inches equals one foot [31.38 cm]).

31. German Leime means, literally, "glues." However, this should probably read Lehm, which means "clay" or "mud."

32. Foot holes were needed to remove broken crucibles and their overflowing contents (Herdglas, hearth glass; see note 37) from the furnace. Lehmann does not explain their use.

33. The sole purpose of this kiln is to fire the crucibles. In a proper glasshouse, however, the lehr is a special device in which the produced glass items are gradually cooled. It remains unclear why Lehmann describes a lehr (Temper=Ofen) when a pottery kiln would seem more appropriate for a smalt factory.

34. Today, the Technische Universität Bergakademie Freiberg in Saxony, Germany.

35. I.e., the master who processes the cobalt blue colorant.

36. Lehmann [note 7] explains Sumpfeschel in v. 1, pp. 68–69: the finest powder residue (Eschel) of the smalt that is collected from sinks (Sümpfe) in the floor of the workshop. Cf. Josef Riederer, "Die Smalte," Deutsche Farben Zeitschrift, v. 22, 1968, pp. 386–395, esp.p. 386.

37. Speiss (Kobaltspeise) is a combination of arsenides resulting from the smelting of various minerals such as cobalt and nickel (Lehmann [note 7], v. 1, pp. 27–28, and v. 2, pp. 104–110). Hearth glass (Herdglas) is glass from a previous melt that fell into the hearth (ibid., v. 1, p. 64). For powder blue (Sumpfeschel), see note 36.

Published on July 19, 2013